Stability of benzotriazolyl-substituted phthalocyanines with respect to thermal oxidative decomposition

Article abstract of DOI:10.1134/S0036024413030345

The thermal oxidative decomposition of benzotriazolyl-substituted phthalocyanines and their copper complexes is investigated by means of thermogravimetric, elemental, and spectroscopic analysis. It is shown that the nature of peripheral substituents exert

Full text of DOI:10.1134/S0036024413030345

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2013, Vol. 87, No. 3, pp. 352–356. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © S.A. Znoiko, V.E. Maizlish, G.P. Shaposhnikov, N.Sh. Lebedeva, E.A. Mal’kova, 2013, published in Zhurnal Fizicheskoi Khimii, 2013, Vol. 87, No. 3,
pp. 371–375.
CHEMICAL THERMODYNAMICS
AND THERMOCHEMISTRY
Stability of BenzotriazolylꢀSubstituted Phthalocyanines
with Respect to Thermal Oxidative Decomposition
,c
S. A. Znoikoa, V. E. Maizlisha, G. P. Shaposhnikova, N. Sh. Lebedevab , and E. A. Mal’kovab
aIvanovo State University of Chemistry and Technology, Ivanovo, 153000 Russia
bG. A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia
cIvanovo Institute of the State Fire Prevention Service, Ministry of Emergency Situations, Ivanovo, 153040 Russia
eꢀmail: znoykosa@yandex.ru
Received February 3, 2012
Abstract—The thermal oxidative decomposition of benzotriazolylꢀsubstituted phthalocyanines and their
copper complexes is investigated by means of thermogravimetric, elemental, and spectroscopic analysis. It is
shown that the nature of peripheral substituents exerts the greatest effect on the thermal stability of the comꢀ
pounds.
Keywords: benzotriazolylꢀsubstituted phthalocyanines, thermal oxidative decomposition, thermal stability.
DOI: 10.1134/S0036024413030345
INTRODUCTION
Thermogravimetric investigations were performed
in air using a thermoanalytical setup that consisted of
a 1000D derivatograph, an instrumentation amplifier
with gain factor of 500; a fiveꢀchannel analogꢀtoꢀdigiꢀ
tal converter and a PC [6]. The heating rate was
5 K/min. A sample of compound 7b was investigated
under a nitrogen atmosphere on a Perkin–Elmer DCS
7 (heating rate, 10 K/min).
Electronic absorption spectra from solutions of the
investigated compounds in organic solvents (chloroꢀ
form, DMF) at concentrations of 10–4–10–6 M were
recorded on a Hitachi Uꢀ2001 spectrophotometer at
~20°C in the wavelength range of 250–900 nm; the IR
spectra were recorded on an Avatar 360 FT–IR ESP in
the area of 400–4000 cm–1 using pellets of KBr. The
1H NMR spectra of 5% sample solutions using 0.1%
TMS as an internal standard were recorded on a
Brucker DRXꢀ500. Elemental analysis was performed
using a Flash EA 1112 CHNS–O Analyzer.
Phthalocyanines and their metal complexes are an
extremely promising class of organic compounds for
practical application [1–3], since they have high therꢀ
mal stability and inherent spectroscopic properties
due to the unique structure of their multicircuit conjuꢀ
gated aromatic system. The presence of substituents in
phthalocyanine molecules gives rise to new valuable
properties and broadens the area of their application,
but can influence their thermal stability. This is particꢀ
ularly important with benzotriazolylꢀsubstituted phthꢀ
alocyanines, since there were no data on the effect of
heterocyclic substituents (particularly the benzotriazꢀ
ole moiety) on the thermal stability of phthalocyaꢀ
nines at the time we were designing our investigation.
Additional interest was also generated by the comꢀ
pounds investigated in this work having thermotropic
mesomorphism (a Cr
→
Mes phase shift at 99–
212 ) [4, 5], further demonstrating the need to study
°
С
their behavior at high temperatures.
RESULTS AND DISCUSSION
The results from our study of the thermal oxidative
stability of benzotriazolylꢀsubstituted phthalocyanines
in the presence of atmospheric oxygen are presented
in Table 1; our thermograms, in Figs. 1 and 2. The
negligible mass loss of investigated samples study was
registered on the TG curves upon heating to 200°C.
The electronic absorption and IR spectra of the invesꢀ
tigated samples recorded before the beginning of the
experiment and after heating to 200°C were identical.
As a consequence, the observed changes are not assoꢀ
ciated with the decomposition processes of phthalocyꢀ
anine molecules.
EXPERIMENTAL
The investigated compounds (1а–4а, 6а) were synꢀ
thesized by the interaction between the corresponding
substituted phthalonitriles and copper acetate at 200–
220
5b
substituted phthalonitriles in the presence of urea at
200–220 for 2 h. The compounds were purified by
column chromatography. The purity of the comꢀ
°
–
С
for 2 h [4]. The metalꢀfree phthalocyanines (3b
,
7b) were synthesized by heating the corresponding
°С
pounds was estimated by the constancy of log
molar extinction coefficient).
ε
(the
352

STABILITY OF BENZOTRIAZOLYLꢀSUBSTITUTED PHTHALOCYANINES
353
N
N
N
R
M = Cu (a), HH (b)
N
O
N N
R =
(1
Y
–5);
(6a, 6b);
X
N
N
N
N
R
N
N
(
7b);
O
N
N
M
N
N
X = S, Y = H (1a); X = O, Y = H (2a);
CH3
CH3
CH3
R
X = O, Y = –C6H5 (3a
,
3b); X = O, Y =
(4a);
C6H5
C6H5
C6H5
N N
X = O, Y =
(5b).
N
N
R
N
N
(
1–7)
Scheme.
The next stage, 200°С to 350–400°С, is characterꢀ occurred in the temperature range of 290–404°C,
corresponding to the removal of the four benzotriazole
substituents according to mass, and is in agreement
with our theoretical calculations of the mass of benzoꢀ
triazole substituents (Table 1).
ized by a substantial change in mass and two peaks on
the DTG and DTA curves (Fig. 1). The 20–30%
reduction in the mass of the sample (Table 1) correꢀ
sponds approximately to the elimination of the four
benzotriazole substituents from the phthalocyanine
molecule. An additional reference of the progress of
thermal decomposition by IR spectroscopy and elemenꢀ
tal analysis supports this assumption. For tetraꢀ4ꢀ(1ꢀbenꢀ
zotriazolyl)tetraꢀ5ꢀ(4'ꢀ(quinoxalylꢀ2')phenoxy)phthaloꢀ
cyanine (7b), it was found by thermogravimetric analꢀ
ysis (TGA) that the first exo peak was registered in the
DTA curve at 290°C; i.e., it was at this point that the
thermal decomposition of this compound began
After sample 7b was heated at 290°C for 10 min, it
was found that 94% of the obtained substance was solꢀ
uble in chloroform, benzene, and DMF (X1); the
character of its electronic absorption spectrum (EAS)
(Fig. 2, curve
curve of tetraꢀ4ꢀ(4'ꢀquinoxalylꢀ2'ꢀphenoxy)phthaloꢀ
cyanine (Fig. 2, curve ). We may therefore assume
4) was analogous to those of the spectral
2
that this stage of thermal decomposition is more likely
related to the elimination of the four benzotriazole
(Fig. 1b). The 23.6% reduction in sample mass substituents. The elemental analysis data of XI are also
Table 1. Thermal oxidative decomposition of benzotriazolylꢀsubstituted phthalocyanines and their copper complexes
No.
R
Δ
m
, %
Δ
T,
°
C
Texo, °C
1a
2a
3a
3b
4a
5b
SPh
31.9 (29.0)
33.4 (29.0)
27.5 (26.6)
28.3 (28.0)
29.9 (28.3)
21.4 (20.7)
360–371
427–455
410–430
460–510
407–413
544–580
366
431
413
503
412
577
OPh
OC6H4(4'ꢀPh)
OC6H4(4'ꢀ ꢀBu)
t
OC6H4(4'ꢀC(Ph)3)
O
6a
6b
29.3 (28.6)
30.3 (28.6)
450–497
480–530
494
520
N
7b
25.1 (23.6)
456–615
542
O
N
Note: Δm is the theoretically calculated value of mass loss at 300–400°C. The experimentally obtained values are in parentheses. ΔT is the
temperature range corresponding to the maximum mass loss of the sample, °C; T is the temperature of the maximum exo effect.
exo
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354
ZNOIKO et al.
m, mg
m
20
, mg
TG
(a)
TG
(b)
10
6
15
10
5
2
0
0
200
400
DTG
600
800
0
200
400
DTG
600
800
U
,
μ
V
U,
μ
V
420
410
400
380
360
370
330
0
,
200
400
600
800
0
200
400
600
800
−
0
U
, μV
−U
μ
V
DTA
DTA
50
40
80
70
90
120
110
0
200
400
600
800
T
0
200
400
600
800
T, C
,
°
C
°
Fig. 1. Thermograms: (a) copper tetraꢀ4ꢀ(1ꢀbenzotriazolyl)tetraꢀ5ꢀphenoxyphthalocyanine (2a) and (b) tetraꢀ4ꢀ(1ꢀbenzotriazꢀ
olyl)tetraꢀ5ꢀ(4'ꢀ(quinoxalylꢀ2')ꢀphenoxy)phthalocyanine (7b).
Table 2. Elemental analysis data on the product (
tained by heating tetraꢀ4ꢀ(1ꢀbenzotriazolyl)tetraꢀ5ꢀ(4'ꢀ
(quinoxalylꢀ2')ꢀphenoxy)phthalocyanine (7b) at 290°C
X) obꢀ
close to those for tetraꢀ4ꢀ(4'ꢀquinoxalylꢀ2'ꢀpheꢀ
noxy)phthalocyanine. The remainder of the heated
sample (X2, ~6%) was soluble in DMF and retained its
phthalocyanine structure (longꢀwavelength absorpꢀ
tion bands at 710 nm in DMF and at 812 nm in conꢀ
centrated H2SO4 were observed in EAS).
Empirical forꢀ
mula
Compound
N, % C, %
Х1
Х2
Х
–
–
–
15.83 74.25
18.69 70.12
16.88 77.35
To confirm our assumption of the removal of the
benzotriazole fragments at the second stage, a sample
of the compound 7b exposed to heating at 290°C
under an inert atmosphere (nitrogen) was investigated
1
by electronic absorption, IR and H NMR spectrosꢀ
Tetraꢀ4ꢀ(1ꢀbenzotriazꢀ
olyl)tetraꢀ5ꢀ(4'ꢀ(quinoxꢀ
alylꢀ2')ꢀphenoxy)phthaloꢀ
C112N28H62O4 21.06 72.25
copy, and elemental analysis. The results are presented
in Figs. 2 and 3 and Table 2. The solubility of the samꢀ
ple heated at the abovementioned temperature in
chloroform and benzene was retained; hence, the oxo
heteryl fragment that ensures solubility in these solꢀ
vents was not eliminated. A 9 nm hypsochromic shift
of the Qꢀband is therefore observed in EAS, and the
ratio between the Qꢀband and its vibrational satellite
cyanine (7b
)
Tetraꢀ4ꢀ(1ꢀbenzotriazꢀ
olyl)phthalocyanine
C56N20H30
28.56 68.57
Tetraꢀ4ꢀ(quinoxalylꢀ2ꢀ
phenoxy)phthalocyanine
C88N16H50O4 16.08 75.85
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STABILITY OF BENZOTRIAZOLYLꢀSUBSTITUTED PHTHALOCYANINES
355
changes (Fig. 2, curve 3). Consequently, the EAS of
the heated sample (X) is identical to that of tetraꢀ4ꢀ
(4'ꢀ(quinoxalylꢀ2'ꢀphenoxy)phthalocyanine (Fig. 2,
676 (
2
)
)
684 (1)
D
curve
2) and differs from that of the initial compound
675 (
3
7b (Fig. 2, curve
1). According to our elemental analꢀ
ysis data, the sample of compound 7b heated at 290°C
contained nitrogen and carbon corresponding approxꢀ
imately to the percentage content of these elements in
tetraꢀ4ꢀ(4'ꢀ(quinoxalylꢀ2'ꢀphenoxy)phthalocyanine
(Table 2, X). The disappearance of the valence vibraꢀ
tion band of the N=N bond in benzotriazole (1040–
1050 cm–1) in the IR spectrum and the signal at
8.71 ppm in the 1H NMR spectrum (the proton labelꢀ
ing for molecules of the compounds is given in Figs. 3a
and 3b) implies that benzotriazole moieties are subject
to elimination. We may thus conclude that the decomꢀ
position of this compound proceeds with the particiꢀ
pation of 1ꢀbenzotriazole fragments at the first stage.
676 (4)
500
600
700
800
, nm
λ
Fig. 2. EAS in DMF: (
quinoxalylꢀ2'ꢀphenoxy)phthalocyanine; (
after heating at 290
1
) compound 7b; (
2) tetraꢀ4ꢀ(4'ꢀ
3) compound 7b
) compound 7b after heatꢀ
Further heating to 360–540°C leads to the thermal
oxidative decomposition of the phthalocyanine macꢀ
°
C (in air); (
4
ing at 290
°
C (nitrogen).
(a)
12
13
11
10
H7, 8
H9
N
8
N
H3H2, 6
7
H11, 12
H1
9
H5
2
1
8
O
N
7
H4
HN
N
N
3
6
4
5
CHCl3
9.5
8.5
7.5
6.5
5.5
δ, ppm
(b)
9
10
8
7
7 H8, 9
H3
H
N
5
N
4
H6
6
H2, 5
2
5
O
H4
4
HN
3
1
CHCl3
9.5
8.5
7.5
6.5
5.5
, ppm
δ
1
Fig. 3. H NMR spectra of tetraꢀ4ꢀ(1ꢀbenzotriazolyl)tetraꢀ5ꢀ(4'ꢀ(quinoxalylꢀ2')ꢀphenoxy)phthalocyanine (7b): (a) initial; (b)
after heating at 290
°
C.
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ZNOIKO et al.
rocycle, which in the case of copper complexes yields
copper oxide. In the case of metalꢀfree compounds,
thermal oxidation is accompanied by the formation of
gaseous products.
Among the investigated copper phthalocyanines,
copper tetraꢀ4ꢀ(1ꢀbenzotriazolyl)ꢀtetraꢀ5ꢀphenylsulꢀ
fanylphthalocyanine (1a) has the least thermal stabilꢀ
ity. Replacing bridged sulfur atoms with oxygen atoms
3b < 6b < 7b < 5b.
It can seen from the presented series that both the
annelation and the introduction of bulky substituents
in the paraꢀposition to phenoxy groups leads to an
increase in the thermal stability of the compounds
(Table 1). Compound 5b containing the bulkiest triphꢀ
enymethyl fragments in the paraꢀpositions to phenoxy
groups have the highest thermal stability.
(
2a) raises the temperature of the maximum exo effect
by 65°C. This is probably due to the difference
between the C–O and C–S bond strengths (a result of
the low C–S bond energy [7]) and the easier oxidation
of the thiobridge owing to the lower electronegativity
of sulfur atom. The high thermal stability of complex
6a is probably due to the bulky 2ꢀnaphthol fragments,
which lie outside the phthalocyanine macrocycle
plane and are turned at an angle of 60° to it, shielding
the phthalocyanine core from oxygen molecules most
efficiently. This raises the temperature of the maxiꢀ
mum exo effect. Analogous regular trends in the subꢀ
stituent’s influence were noticed earlier when analyzꢀ
ing the stability of benzotriazolylꢀsubstituted phthaloꢀ
cyanines [8].
The thermal oxidative decomposition of most of
the studied metalꢀfree phthalocyanines takes place at
higher temperatures than for the corresponding metal
complexes. As can be seen from the data presented in
Table 1, their thermal stability depends substantially
on the nature of the substituent introduced in the
orthoꢀposition to the 1ꢀbenzotriazolyl fragment. The
temperature of the maximum exo effect of phthalocyꢀ
anines–ligands thus rises in the following series
(Table 1):
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